1. Field of the Invention
The invention relates to a liquid crystal display device and a method of fabricating the same, and more particularly to an in-plane switching type liquid crystal display device in which a voltage applied across a pixel electrode and a common electrode both formed on a thin film transistor (TFT) substrate causes liquid crystal to rotate in a plane substantially in parallel with the substrate, and a method of fabricating the same.
2. Description of the Related Art
An active matrix type liquid crystal device including a thin film transistor (hereinafter, abbreviated as “TFT”) as a device for switching a pixel provides high image quality, and hence, is currently widely used, for instance, as a monitor display of a desk-top type computer.
An operation mode of a liquid crystal display is generally grouped into a twisted nematic mode in which aligned liquid crystal molecules are rotated such that directors thereof are directed in a direction perpendicular to a glass substrate, and an in-plane switching hereinafter, abbreviated as “IPS”) type liquid crystal display device in which aligned liquid crystal molecules are rotated such that directors thereof are directed in a direction parallel with a glass substrate.
In IPS liquid crystal display device, pixel electrodes and common electrodes are formed in parallel with each other on a first transparent substrate on which TFT is also fabricated. A voltage is applied across the pixel and common electrodes to thereby generate a field in parallel with the substrate. Directors of liquid crystal molecules are rotated in accordance with the thus generated field, and as a result, it is possible to control an amount of light passing through a liquid crystal layer.
Accordingly, since directors are rotated in a plane in parallel with a substrate in the IPS liquid crystal display device., the IPS liquid crystal display device will not be accompanied with a problem unlike TN type liquid crystal display device that a relation between an amount of light passing through a liquid crystal layer and a voltage applied across a pixel electrode and a common electrode is not uniform between a case wherein the device is viewed from the directors and a case wherein the device is viewed from a direction of a normal line of the substrate. As a result, the IPS liquid crystal display device ensures high quality images in a wide visual angle.
In the IPS liquid crystal display device, a liquid crystal layer is in homogenous alignment, and is sandwiched between two polarizing plates having polarizing axes perpendicular to each other. One of the polarizing axes is directed in an alignment direction of liquid crystal molecules. Thus, in general, when no voltage is applied across a pixel electrode and a common electrode, a display screen displays black, whereas when a voltage is applied across a pixel electrode and a common electrode, liquid crystal molecules are twisted in accordance with a field, to thereby cause a display screen to display white. The IPS liquid crystal display device is widely used, because it can stably keep a brightness low when a display screen displays black.
However, the conventional IPS liquid crystal display device is accompanied with a problem, as illustrated in FIG. 1, that since a pixel electrode 7 and a common electrode 3 are both formed in the form of a line, and liquid crystal molecules are rotated only in a single direction, a display screen is colored when the display screen is obliquely viewed while the display screen displays white.
In order to solve the problem, Japanese Unexamined Patent Publication No. 9-311334 has suggested a liquid crystal display device in which the pixel electrode 7 and the common electrode 3 are designed to be bent in the form of “<” in a pixel.
FIG. 2A is a plan view of the liquid crystal display device suggested in the above-mentioned Publication, and FIG. 2B is a cross-sectional view taken along the line IIB—IIB in FIG. 2A.
The illustrated liquid crystal display device is comprised of a first transparent substrate 1 on which a thin film transistor (TFT) is fabricated, a second transparent substrate 11 on which a color filter is formed, and a liquid crystal layer 17 sandwiched between the first and second transparent substrates 1 and 11.
Gate electrodes 2 and data lines 6 are formed on the first transparent substrate 1 such that they are substantially perpendicular to each other, and TFTs 4 are fabricated in a matrix at intersections of the gate electrodes 2 and the data lines 6. In each of pixels, pixel electrodes 7 and common electrodes 3 are alternately formed in parallel with each other. The pixel electrodes 7 and the common electrodes 3 are designed to have a bending point at which the pixel electrodes 7 and the common electrodes 3 are bent in the form of “<”.
On the second transparent substrate 11 are formed a black matrix layer 12 for interrupting a light to pass therethrough, a color layer 13 for displaying red, green and blue (RGB) colors, and a planarizing film 14 which covers the black matrix layer 12 and the color layer 13 therewith and has a planarized surface.
Alignment films 18 are coated on surfaces of the first and second transparent substrates 1 and 11 such that the liquid crystal layer 17 is sandwiched between the alignment films 18. Liquid crystal in the liquid crystal layer 17 is homogeneously aligned substantially in parallel with a direction in which the data lines 6 extend.
A polarizing plate 16a is adhered to an outer surface of the first transparent substrate 1, and a polarizing plate 16b is adhered to an outer surface of the second transparent substrate 11. The polarizing plates 16a and 16b have polarizing axes perpendicular to each other, and one of the polarizing axes is set in parallel with a direction in which liquid crystal molecules are aligned.
By applying a voltage to the pixel electrode 7 through TFT 4 to thereby establish a horizontal field between the pixel electrode 7 and the common electrode 3, liquid crystal in the liquid crystal layer 17 are twisted in a plane parallel with the first and second transparent substrates 1 and 11, to thereby control display.
In the liquid crystal display device illustrated in FIGS. 2A and 2B, when a voltage is applied across the pixel electrode 7 and the common electrode 3, fields are generated in different directions from each other in two regions sandwiching the bending points of the pixel and common electrodes 7 and 3 therebetween, as illustrated in FIG. 4 with arrows X1 and X2, Accordingly, liquid crystal molecules in the liquid crystal layer 17 are twisted in two different directions. As a result, different colors can be seen in the two regions in a display screen when the display screen is obliquely viewed while the display screen displays white. Since the colors compensate for each other, the above-mentioned different colors can be reduced.
As mentioned above, it is possible in the IPS liquid crystal display device to widen a visual angle and reduce color in a display screen by designing the pixel electrode 7 and the common electrode 3 in the form of “<”. However, since it is necessary in the IPS liquid crystal display device to form both the pixel electrodes 7 and the common electrodes 3 such that a numerical aperture in a pixel is kept high, it would not be possible to arrange the pixel electrodes 7 and the common electrodes 3 in a high density, and hence, it would be also impossible to reduce a gap between adjacent electrodes. As a result, a greater voltage has to be applied across the pixel electrode 7 and the common electrode 3 in order to establish a field having a higher intensity, resulting in an increase in power consumption.
On the other hand, if a smaller voltage is applied across the pixel electrode 7 and the common electrode 3, a resultant field would have a smaller intensity, causing a problem that liquid crystal molecules in the liquid crystal layer 17 cannot quickly respond to the field.
One of solutions to this problem is to select liquid crystal molecules having a small viscosity. Such liquid crystal molecules make it possible to respond to a field when a display screen displays white. However, such liquid crystal molecules is accompanied with a problem that an effectively small field is applied to the liquid crystal layer 17 in half tone, and hence, the liquid crystal molecules responds to a field twice slower than a case in which a display screen displays white.
Hereinbelow is explained this problem with reference to FIG. 3. FIG. 3 illustrates a relation between a voltage applied to liquid crystal and both a transmissivity of a panel and a response. In FIG. 3, a broken line shows a relation between an applied voltage and a transmissivity of a panel, and a solid line shows a relation between an applied voltage and a response.
As illustrated in FIG. 3, as a voltage applied to liquid crystal increases, a direction in which liquid crystal is aligned is twisted towards a direction of a field, and resultingly, a light is more likely to pass through the liquid crystal layer 17. When a certain voltage is applied to the liquid crystal, the liquid crystal is twisted by 45 degrees, and the transmissivity is in maximum. Namely, a display screen displays white.
When a greater voltage is applied to the liquid crystal, the liquid crystal molecules can more readily rotates. Hence, as a greater voltage is applied to the liquid crystal, the liquid crystal can respond to a field more rapidly. Since a smaller voltage is applied to the liquid crystal in a half tone display state than in a white display state, the liquid crystal would respond to a field more slowly in a half tone display state than in a white display state.
In general, a threshold voltage Vth defined as a minimum voltage for driving liquid crystal is represented in accordance with the following equation (1).Vth=π×L/d×(K22/ε0Δε)1/2  (1)
In the equation (1), L indicates a distance between the pixel electrode 7 and the common electrode 3, “d” indicates an effective cell gap, K22 indicates a twist elastic coefficient of liquid crystal existing in the liquid crystal layer 17, and Δε indicates anisotropy in a dielectric constant.
As is obvious in the equation (1), the greater the distance L is, the smaller an intensity of a field is, and hence, the greater the threshold voltage Vth is. On the other hand, the greater the cell gap “d” is, more readily the liquid crystal can be made to rotate, and resultingly, the smaller the threshold voltage Vth is. Accordingly, in order to increase a response speed of the liquid crystal, it is necessary to reduce the distance L, and/or increase the cell gap “d”.
However, though the threshold voltage Vth can be made smaller by uniformly narrowing the distance L between the pixel electrode 7 and the common electrode 3 or by uniformly increasing the cell gap “d”, it would be still impossible to accomplish quick response in half tone when a small voltage is applied across the pixel electrode 7 and the common electrode 3.
In addition, if the distance L were made smaller, there would be caused a problem that the electrodes in a pixel would occupy a larger area, resulting in reduction in a numerical aperture.